Shale oil deposits can be found generally in rock shallow layers near to soil surfaces, and are regarded as valuable raw materials of the chemical industry primarily with regard to their olefin, aromatic hydrocarbon and asphaltene content.
A characteristic feature of kerogen is that it is hardly soluble, if at all, in the majority of the known and utilized organic solvents, like gasoline or naphtha. The extraction of kerogen is also difficult since it adheres firmly to the reservoir rocks and cannot be desorbed easily. The same relates to tars occurring in young coal ranks.
Various methods have been described in the patent literature for the recovery of oil from oil shales. Some of these known methods require the mining, crushing and retorting of the oil-containing rocks, whereas in other methods oil is recovered by processing, i.e. by retorting or extracting.
Thus, for example, it is known to recover kerogen from oil shale by retorting, which can be performed like the swealing of coal. In this method oil shale is heated generally to 700.degree.-800.degree. K., whereas coal is heated generally to a temperature of about 1000.degree. K. Hydrocarbons undergo thermal decomposition at these temperatures. The bulk of the gas, obtained upon cooling the thermal decomposition products, consists of methane and hydrogen beside nitrogen and which is utilized as fuel. The quality of the liquid components obtained in this method does not reach the required level.
As a further disadvantage, retorting requires much heat energy, and the process can be maintained only by combusting a substantial part of the gas formed. In general, a substantial pat of oil shale should be combusted in order to heat oil shale to 800.degree. K. and to attain the thermal dissociation of kerogen. This is the theoretical limit which determines the quality of the raw material worth processing. In practice only oil shales with a kerogen content greater than 8-10% are regarded as substances worth processing; the known and operating plants are based generally on oil shales with kerogen contents of 16 to 60%. The low heat conductivity of the rock also impedes the technical realization of retorting.
The method described in U.S. Pat. No. 4,060,479 is essentially based on dry distillation. U.S. Pat. No. 4,054,505 describes a method for the separation of oil from oil shales or bituminous sands; according to this method bitumen is isolated by the simultaneous application of solvent treatment and ultrasonic irradiation.
U.S. Pat. No. 4,054,506 relates to the improvement of the above method; in this latter method mechanical stirring of the mixture is also applied.
U.S. Pat. No. 4,067,796 describes a method for the isolation if bitumen based on the separation of an aqueous solution from organic solvents.
The so-called "coal oil" present in young coal ranks, such as lignite, and brown coal can be extracted with toluene at 350.degree. to 400.degree. C. temperature and 100 to 200 bar pressure, under supercritical conditions (Erdol und Kohle, Erdgas, Petrochemie, pp. 314-326; 1980, 07.).
The method describes in the U.S. Pat. No. 4,067,616 can be regarded essentially as a transition between the in situ isolation and excavation methods, since according to the process the viscosity of kerogen is lowered with appropriate solvents and then kerogen is brought to the surface, without removing, however, the rocks covering the oil-containing rock blanket. A characteristic feature of the methods for the in situ isolation of kerogen is that the viscosity of kerogen present in the carrier rock is lowered in order to enhance the outflow of kerogen.
U.S. Pat. No. 4,045,085 describes an apparatus for the in situ isolation of oil from oil shales, utilizing retorts for fracturing oil shale. In the method described in U.S. Pat. No. 4,066,172 bitumen is isolated from the underground reservoir rocks by flowing a hot medium through the rock.
It has also been suggested to heat the reservoir rocks by electromagnetic waves in order to separate oil from oil shales. Such methods are described e.g. in French Pat. No. 1,260,264 and in U.S. Pat. No. 2,257,738. The essence of these methods is that the kerogen-containing rock is heated by radiofrequency irradiation at a wavelength which enables the energy to enter the material and to convert to heat energy directly in the material to be isolated.
The solution described in French Pat. No. 1,288,621 relates to the improvement of the above methods. According to this latter reference heat energy and high frequency electric energy are generated simultaneously in the bore formed in the oil-containing rock, whereupon the viscosity of the hydrocarbons lowers and the hydrocarbons can be extracted.
U.S. Pat. No. 3,104,711 describes a high frequency heating apparatus mounted in the production well, which enables reduction in the dielectric losses caused by the water accumulated in the bore.
The methods summarized above have not become generally acceptable, since a great deal of the electric energy introduced is absorbed in water, a substance with extremely high dielectric loss, and is consumed for the heating and evaporation of water. Further losses of energy arise from the heating of the rock; thus only a small fraction of the energy introduced serves the required purpose. As known, the distribution of energy absorption is proportional to the product of the dielectric coefficient and the mass share of the medium concerned, thus the energy fraction which serves the isolation of oil is relatively low.
The efficiencies of the above methods, as well as the recovery ratio of the hydrocarbons provided by them do not reach the theoretical optimum, the methods are uneconomical, thus they could not attain general use in practice.